CHAPTER XX
Butterfly resemblances—A living leaf—How spiders trap butterflies—Butterfly doubles—Suggested explanation—More evidence wanted—Warning coloration—A theory on trust—A straightforward test—Advice to naturalists—A strange omission.
SOME of the most remarkable instances of protective resemblances amongst insects are exhibited by butterflies, one, perhaps, being the most perfect existing under nature; however, I only say perhaps. This is the world-renowned leaf butterfly of Sumatra, and elsewhere in the Malay Archipelago. Of the great purple emperor family, it is purple on the upper surface, and gleams like a meteor as it shoots about in the rich, sun-bathed atmosphere of the tropics, its conspicuousness being enhanced by a sort of miniature, sharp-pointed swallow-tail, in which the hinder pair of wings end, and a broad, orange bar, like a sash or scarf of honour, running right across the anterior wings. It flies boldly and strongly, and when it descends upon a bush or shrub it is as though a little purple torch had shot through the foliage; but all at once, even though you see it come down just in front of you, it has vanished utterly—the torch has gone out. You may look and look, but unless you know the trick, and have seen the settling, and never taken your eyes off the exact spot, you will never find thebutterfly, or see anything more of it until, all at once, it gleams in the air again. For the under part of the leaf butterfly’s lovely purple wings is like the leaf indeed—“the sere, the yellow leaf”—with a midrib running down the centre veinings on either side, a curled tip at the top, a stalk at the bottom, and everything proper to leaves, but not as a rule to butterflies. All four wings join in this effect, for being thrown up in the usual way when the insect settles, the leaf-like shape is thus brought about, one-half of the under surface being seen on each side in clear profile, whilst the purple now lies hid within, like the pictures on a folded screen. As for the body of the butterfly, that is hidden inside the wings too; the legs are all but invisible, and the two little pointed swallow-tails, just touching the plant’s stem with their mutual tip, make the stalk of the leaf. Even on the wall of a room or a curtain it would seem as though a dead leaf were sticking there; how much more when, as is always the case, the butterfly flies into some bush or thicket crowded with dry, brown leaves, and settles all amongst them. It is not that you don’t see it there that makes you miss it, but that you see it and scores of brown leaves all about it, every one of which looks just the same as itself.
PROTECTIVE MIMICRY
PROTECTIVE MIMICRY
PROTECTIVE MIMICRY
The picture at the top shows birds pursuing butterflies, while in the one below the same birds have lost their prey, as the butterflies have alighted and show only the underside of their wings, which are practically indistinguishable from the neighbouring leaves.
To make the matter plainer, in case this is not a very accurate description, here is the account of an eye-witness: “This species,” says Dr. Wallace, “was not uncommon in dry woods and thickets, and I often endeavoured to capture it without success, for after flying a short distance it would enter a bush among dry or deadleaves, and however carefully I crept up to the spot, I could never discover it till it would suddenly dart out again, and then disappear in a similar place. At length I was fortunate enough to see the exact spot where the butterfly settled, and though I lost sight of it for some time, I at length discovered that it was close before my eyes, but that in its position of repose it so closely resembled a dead leaf attached to a twig as almost certainly to deceive the eye, even when gazing full upon it.” Then follows a minute explanation of the imposture. “The end of the upper wings terminates in a fine point, just as the leaves of many tropical shrubs and trees are pointed, while the lower wings are somewhat more obtuse, and are lengthened out into a short, thick tail. Between these two points there runs a dark, curved line exactly representing the midrib of a leaf, and from this radiate on each side a few oblique marks, which well imitate the lateral veins. The tint of the under surface varies much, but it is always some ashy brown or reddish colour, which matches with those of dead leaves. The habit of the species is always to rest on a twig and among dead or dry leaves, and in this position, with the wings closely pressed together, their outline is exactly that of a moderately-sized leaf, slightly curved or shrivelled. The tail of the hind wings forms a perfect stalk, and touches the stick, while the insect is supported by the middle pair of legs, which are not noticed among the twigs and fibres that surround it. The head and antennæ are drawn back between the wings, so as to be quite concealed, and there is a little notch hollowed out at the very base of thewings, which allows the head to be retracted sufficiently. All these varied details combine to produce a disguise that is so complete and marvellous as to astonish everyone who observes it, and the habits of the insect are such as to utilise all these peculiarities, and render them available in such a manner as to remove all doubt of the purpose of this singular case of mimicry, which is undoubtedly a protection to the insect. Its strong and swift flight is sufficient to save it from its enemies when on the wing, but if it were equally conspicuous when at rest it could not long escape extinction, owing to the attacks of the insectivorous birds and reptiles that abound in the tropical forests.”[104]
Dr. Wallace then speaks of another closely allied species which is common in India, on the under surface of whose wings there are sometimes, to the boot of all that has been described, in the way of disguise, “patches and spots formed of small black dots, so closely resembling the way in which minute fungi grow on leaves that it is almost impossible, at first, not to believe that fungi have grown on the butterflies themselves.”[104]The minuteness of a resemblance like this is really very surprising, for it seems as though the butterfly-hunting bird or insect—some powerful wasp may represent the latter—was capable of minutely examining the object in question, and saying to itself, as it were, “I don’t think that can be a leaf, because there are no black spots upon it,” orvice versâ. In reality, however, it is no doubt the general effect, to which every detail contributes, that tells. What such resemblances do seem to me to show—and this, I think, is a newidea—is the accuracy and precision of some insects’ sight. How insects see things has long been a question, and many, I suppose, think it quite uncertain whether a leaf, for instance, throws the same picture on their retina that it does on ours. But if, to deceive them, the copy must be such that it also deceives us, is it not clear that it does? Otherwise the effect of the original could probably be reproduced by a less accurate copy. How little, after all, does the finest painting really resemble nature! The effect alone does so, not the means by which it is arrived at. Surely, then, if an insect, looking at a leaf or any other object, received but a general impression of colour, with an outline more or less blurred, or ill-defined, these copies of nature by nature—made to deceive—would bear witness to the fact. A study of protective resemblances is perhaps the best way of forming an idea as to how creatures, other than ourselves, see the world. It is even possible that such resemblances exist, which we, because we see things differently, are totally incapable of detecting.
I do not know if any other striking case of resemblance to an inanimate object (if plant life can be included under this term) is offered by the butterfly world, though there are several more of the same kind, but I cannot remember one just now. No doubt there are many which have not yet been discovered. We have, however, various instances of concealment even here in England, as, for instance, the peacock butterfly; but these, as well as special resemblances, are, for the most part, more marked in moths. The lappet moth, indeed, though it does not quite getthe shape, looks very like a dead brown leaf, whilst in the buff-tip moth we are supposed to have a special resemblance to a piece of rotting wood, clothed with moss or lichen, and broken at each end. Personally, I have never received the impression of such a definite object, but only a general one of rot and decay. Even here, however, I do not believe I could ever be taken in, for the yellow head and tips of the wings, which are supposed to offer a perfect resemblance to the two broken ends of the piece of wood, are to me the tell-tale parts, and instantly cry out, “Moth!” In fact, soft as is the colouring of the buff-tip, it still seems to me a salient object, and I do not think very much of that bird’s eyesight who fails to detect it under anything like favourable circumstances.
Another moth that flies by day, and is not uncommon in the United States, bears, when sitting on a leaf, a much stronger resemblance to a bird-dropping, but in this not uncommon form of imitation moths, and all other insects, are outdone by spiders, who use it aggressively against them, and particularly, it would seem, against butterflies, as the following instances will show. Mr. Forbes once, whilst travelling in Java, saw a butterfly settled upon a bird-dropping. He watched it for some time, and then, wondering at its long stay, approached cautiously, and, slowly extending his hand, actually caught it by its wings, between his finger and thumb—no mean feat, as it seemed, yet there was nothing to boast of. As he lifted the butterfly only the wings came away, the rest of it staying with the supposed bird-dropping,which was now seen to be a spider, who, having caught the butterfly by means of this shameful imposture, was quietly occupied in eating it. The disguise in this case was of the most wonderful perfection. “Such excreta,” says Mr. Forbes, who discovered this one, “consist of a central and denser portion of a pure white, chalk-like colour, streaked here and there with black, and surrounded by a thin border of the dried-up, more fluid part.”[105]The appearance of each of these constituent parts was successfully counterfeited by the spider in question, who, in its own person, represented the more solid material, and spun the rest with its web.
As I know from early experience, when a naturalist makes a prize, all at once, of some interesting specimen, for some time afterwards he expects, or, rather, feels as if he would see some other on every leaf or twig; but time went by and no more of these “vain, delusive” spiders presented themselves. At length, years afterwards, the same naturalist found himself by the banks of the Moesi river, in Sumatra (which sounds much more interesting than the Thames, for instance), and this was his second experience. “I was,” he says, “rather dreamily looking on the shrubs before me, when I became conscious of my eyes resting on a bird-excreta-marked leaf. How strange, I thought, it is that I have never got another specimen of that curious spider I found in Java, which simulated a patch just like this! I plucked the leaf by the petiole while so cogitating and looked at it half-listlessly for some moments, mentally remarking how closely that other spider had copied nature, when, to my delighted surprise,I discovered that I had actually secured a second specimen, but the imitation was so exquisite that I really did not perceive how matters stood for several moments. The spider never moved while I was plucking or twirling the leaf, and it was only when I placed the tip of my little finger on it that I observed that it was a spider, when it, without any displacement of itself, flashed its falces into my flesh.”[105](He means it bit him.)
Not all butterflies are entrapped by the kind of simulacrum here noticed. Nature can adapt herself to every taste, and in South Africa there are spiders who make themselves attractive by appearing to be flowers. Of some of these and theirmodus operandiMr. Rowland Trimen, who was curator of the Cape Town Museum, gives the following interesting account: “Many species of spiders,” he says, “are well adapted to succeed by being coloured in resemblance to the flowers in or on which they await the arrival of their victims. One that inhabits Cape Town is of the exact rose-red of the flowers of the oleander, and, to more effectually conceal it, the palpi, top of the cephalothorax, and four lateral stripes on the abdomen, are white, according remarkably with the irregular white markings so frequent on the petals ofNerium.”[106]These, indeed, must be beautiful spiders, and one would like to hear a little more of them, but Mr. Trimen goes at once from red to yellow. “I was led,” he continues, “to notice a yellow spider of the same group in consequence of seeing that two of a number of butterflies on the flowers ofSenecio pulugeradid not, on my approach, fly off with their companions. Each of theseunfortunates turned out to be in the clutches of a spider, and when I released them I observed their captors very narrowly, and I found that the latter’s close resemblance to theSenecioflowers was not one of colour alone, but due also to attitude. This spider, holding on to the flower stalk by the two hinder pairs of legs, extended the two long front pairs upward and laterally. In this position it was scarcely possible to believe that it was not a flower seen in profile, the rounded abdomen representing the central mass of florets and the extended legs the ray florets, while to complete the illusion the femora of the front pair of legs, adpressed to the thorax, have each a longitudinal red stripe, which represents the ferruginous stripe on the sepals of the flower.”[106]
Later on, Mr. Trimen was so fortunate as actually to see a butterfly caught by another flowery impostor:—“The butterfly,” he tells us, “was engaged in honey-sucking on a white flower-head ofLantana, and explored each individual flower with its proboscis. While I was watching it, the butterfly touched and partly walked over what looked like a slightly folded or crumpled flower about the middle of the cluster. This turned out to be a spider, which instantly seized the butterfly, throwing forward its front legs, somewhat after the fashion of a mantis. In this spider the effect of the little depressions on the limb of the corolla was given by some depressed lines on the back of its smooth white abdomen.”[106]
Other spiders resemble snail-shells, others ants, and one, at least, is like a small scorpion, but we will return to the butterflies. As I have said, except for that wonderfulcopy of a leaf, already described, I cannot think of any very extraordinary resemblances amongst them, belonging to that class, but there are others which form a little class of their own. In the last chapter we have seen bees imitating bees, and in this we will make the acquaintance of certain butterflies which, as it were, pretend to be of a species which they do not really belong to. Thus in Brazil, by the great River Amazons, a number of large showy butterflies are found which belong to the family of theHeliconea, and wherever these fly they are accompanied by various other butterflies, belonging to quite different families, which are nevertheless so extremely like them that even Mr. Bates, who, for eleven years, ran up and down the Amazons with a butterfly-net in his hand, could never be quite sure which kind it was that he was going to catch. Often, when he thought he had got aHeliconeahe was perfectly thunderstruck to find it was really aPapilio,Pieris,Euterpe,Leptalis,Protogonius,Ithoneis,Dioptis,Pericopis,Hyelosia, or something of that sort; or again, when it was one of these he was after, and at last he thought he had it in the net, he would be petrified, on looking more closely, to find that what he had really caught was aHeliconea.
But now, as all these butterflies were alike or nearly alike, how could Mr. Bates tell—or how had anybody been able to tell before him—that they were really not all the same species—that aHeliconeawas not aPapilio, or that aPapilio,Pieris,Euterpe, etc., were not all of themHeliconeas? This, at first sight, seems a difficult question to answer, but really it is not, because, in allthese families of butterflies, the various species composing them bear a kind of generalised resemblance to one another: there is a family likeness, in fact, and this is not only the case in regard to their outward appearance—the shape and colour of their wings, etc.—but it applies, in a still greater degree, to their structure and internal economy. Thus, however strongly aPieris, or one of those others, might resemble aHeliconea, the trained eye of an entomologist could easily see that it really was a member of another family, and since, in resembling theHeliconea, it departed from the general type of pattern and colouring exhibited by the family to which it belonged, whilst this one species ofHeliconeait resembled was like the others, it might be inferred that the latter was the imitated and not the imitating form. Again—and this is still more decisive evidence in the cases where it applies—the resemblance is often confined to one sex of the copying species, viz. the female, so that whilst she is hardly to be distinguished from the model on which she has founded herself, the male retains the appearance, together with all the other characteristics, of the race to which both he and she belong.
But now came a further question, the most puzzling or, at any rate, the most important one of all, viz. Why should the one butterfly imitate, or rather resemble the other, in such an extraordinary degree—a degree seeming to preclude the possibility of mere chance having brought it about? This question Mr. Bates is supposed to have been the first to answer, though I cannot help thinking, myself, that he has only extended an explanation, which,in some cases, was so obvious that no one had thought of pointing it out, to these other cases where it was not nearly so easy to see. For what can be plainer, as Mr. Bates himself remarks, than that a moth, for instance, by closely resembling a hornet, would escape the attacks of birds that might otherwise have devoured it? I cannot think but that so patent an explanation had been in the minds of many long before 1862, and though no one previous to that date may have applied the principle of natural selection to such cases, it must be remembered that natural selection had been established by Darwin some ten or twelve years before.
Bates, however, besides making an ingenious application of the above principle to a special case, gave a real reason for something which was not at all obvious, viz. why one butterfly should be a gainer by closely resembling another; and this no one had hitherto been able to do. His surmise, which has since in many instances been confirmed, is as follows. Having first pointed out that theHeliconeabutterflies are a numerous, flourishing race, whilst those species that imitate them are poor in numbers, he says, “What advantages theHeliconidæpossess to make them so flourishing a group, and, consequently, the object of so much mimetic resemblance, it is not easy to discover. There is nothing apparent in their structure or habits which could render them safe from persecution by the numerous insectivorous animals which are constantly on the watch in the same parts of the forest which they inhabit. It is probable they are unpalatable to insect enemies. Some of them have glands near the endof the abdomen which they protrude when roughly handled; it is well known that similar organs in other families secrete fetid liquids or gases and that these serve as a protection to the species. They have all a peculiar smell. I never saw the flocks of slow-flyingHeliconidæin the woods persecuted by birds or dragon-flies, to which they would have been easy prey; nor when at rest on leaves did they appear to be molested by lizards, or the predaceous flies which were very often seen pouncing on butterflies of other families. If they owe their flourishing existence to this cause it would be intelligible why species whose scanty number of individuals reveals a less protected condition, should be disguised in their dress, and thus share their immunity. Is it not probable, seeing the excessive abundance of the one species and the fewness of individuals of the other, that theHeliconeais free from the persecution to which theLeptalisis subjected?”[107]
No sooner was this suggestion made than naturalists all over the world began to test it, or rather to say that it ought to be tested. Some experiments have been made, but they have not been very numerous, and it can hardly be said that they entirely support Bates’s view. Sometimes they do and sometimes they do not, so as there is no reason to suppose that every butterfly is relished by every kind of insect-eating creature, this is not conclusive, till the same tests are employed in regard to butterflies that are not imitated in this way; for if the latter have not been imitated on that account, it need not be on that account that others have been imitated. Thus Belt says, as the result of his observations, “TheHeliconidæare distasteful to most animals; I have seen even spiders drop them out of their webs again; and small monkeys, which are extremely fond of insects, will not eat them, as I have proved over and over again.”[108]He also “observed a pair of birds that were bringing butterflies and dragon-flies to their young, and although theHeliconidæswarmed in the neighbourhood, and are of weak flight so as to be easily caught, the birds never brought one to their nest.”[109]This seems very good evidence of the truth of Bates’s theory, but then, as against it, we learn from the same observer that “another spider that frequented flowers seemed to be fond of these very same butterflies,” and as to the spiders which were seen to drop them out of their webs, they may resolve themselves into one, since farther on Belt says, “A large species of spider also used to drop them out of its web when I put them into it.”[109]Then we are told that “there is, however, a yellow and black-banded wasp that catches them to store his nest with”; and which, having done so, “would quietly bite off its wings, roll it up into a ball, and fly off with it.”[110]Professor Poulton calls these cases “interesting exceptions,” and easily accounts for them. But might not further observation keep adding to the number of exceptions, until at last, they become so numerous that all one could say would be this: “There is a great choice of insects in tropical America, and some creatures may prefer one kind and some another, to whatever species they belong.” In India, again, where there is another family of butterflies having doubles, or understudies, only one species was refused by all the mantids which a Frenchnaturalist gave them to. Others were eaten by all of them.
Has any man tried eating one of these butterflies? That was what Professor Wheeler did to test another supposed case of the same sort. Here the insect was a large and very conspicuously coloured day-flying moth. This moth has not an understudy, as far as is known, but it was supposed, then, to be an example of what is called “warning coloration,” that is to say, its bright colours were believed to be flourished in the face of any and every animal it might meet with, in order to warn them that it was not good to eat. Otherwise some bird, or lizard, or other creature might kill it before it had time to find out that it wasn’t. For instance, had it been just a brown moth—there are so many of these and most of them good to eat—how was it to be distinguished from others? But such a get-up asthat—black and white wings and a black and orange body—once seen it was not to be forgotten. It was like the red flag at a rifle range, warning one off, and this is the theory of warning coloration. So Professor Wheeler, as he rode through the deserts of Wyoming, with the moths all about him, resolved to test this theory which had lived for a long time, and still goes on living a good deal on trust. “He dismounted from his horse and proceeded to masticate the body of one of the moths. To his astonishment the little flavour that it contained was mild and pleasant, one may almost say nut-like.”[111]Perhaps it may be thought that, on the “de gustibus” principle, what is pleasant to a human being might be disagreeable to a bird or a lizard; but Professor Wheeler tried anotherexperiment. “Another day-flying moth, common in our eastern States, has deep black wings, each adorned with a pair of large yellow spots, and there is a dash of orange on its legs. It certainly cannot be a mimetic species (if it were, of course, one would not expect it to be nasty) as there is no other day-flying moth which could serve as its model. Several of these moths were given to some lizards that had previously been well fed on house-flies and could not, therefore, be very hungry. The moths were seized at once, and devoured, with evident signs of relish.”[111]
As a result of these experiments Professor Wheeler concludes that “naturalists should be more careful in imputing nauseous or disagreeable qualities to some conspicuously coloured animals,” and he suggests that “if every field entomologist could only bring himself to repeat the writer’s experiment on one of many cases of ‘flaunted nauseousness’ and place his taste impressions on record, we should in the course of time have a really valuable body of evidence, for we can hardly assume that beasts, birds, and reptiles can find things ‘nauseous’ which are quite tasteless, or even pleasant, to the human palate.”[111]“Il n’y a pas de réplique à cela,” and how it is that so simple a plan did not occur to Mr. Bates during all the eleven years he was on the Amazon it is not very easy to imagine. On the whole, perhaps, it may be said that the reason why certain butterflies are imitated by other butterflies has not been so satisfactorily settled as the fact that they are so imitated. But, on the other hand, there is some—perhaps much—evidence of the truth of Bates’s theory, and, moreover, that theory is initself so plausible that it seems to require a good deal of evidence to overthrow it.
It is not only in South America that butterflies dress up like one another. Instances similar to those here given occur also in Africa and the Malay Archipelago, as well as in other parts of the world. There is even one doubtful case in England, both the copy and original being moths. Moths, especially the day-flying ones, are represented in these phenomena as well as butterflies, which are sometimes imitated by them.